Interactions between adjacent protein molecules may be reciprocally linked to events taking place within those protein molecules. An example of such a reciprocally-linked function is the interaction between liganding of the heme groups of sickle cell hemoglobin and the tendency of the molecules to undergo intermolecular aggregation. Considerations of conservation show that the effect must be reciprocal: liganding of the heme favors dissociation of the aggregate; intermolecular aggregation favors dissociation of the ligand. We have recently encountered concentration-linked, presumably reciprocal, functions in three monomeric, oxygen-reactive hemeproteins: cytochrome c peroxidase, horseradish peroxidase and leghemoglobin. We propose to study the large, concentration-dependent changes in optical and EPR spectra, in the kinetics and equilibria of ligand binding and in redox properties which we find to occur in cytochrome c peroxidase and in horseradish peroxidase. Analytic methods employed mainly will be static optical spectrophotometry and stopped-flow kinetic spectrometry.